Carbon dioxide recirculation

ABSTRACT

Carbon dioxide recirculating apparatus ( 20, 120 ) is disclosed for use in an arrangement having combination means ( 115 ) and a path for the flow of a gas through the combustion means ( 115 ). The apparatus ( 20, 120 ) comprises extraction means ( 221 ) for extracting carbon dioxide from a first region of the path downstream of the combustion means ( 115 ). It further includes condensing means ( 26, 30 ) for condensing the extracted carbon dioxide, and feed means ( 36, 136 ) for feeding the condensed carbon dioxide to a second region of the path upstream of the combustion means.

This invention relates to carbon dioxide recirculating apparatus. Moreparticularly, the invention relates to carbon dioxide recirculatingapparatus for heat engines, for example gas turbine engines, or fuelcells.

It is known to inject a water based fog into the compressor region of agas turbine engine to increase the power of the engine. The water isatomised when it is sprayed into the compressor region and forms a fog.The water droplets forming the fog vaporise and extract latent heat ofevaporation from the gases in the compressor, thereby cooling thesegases. This has a beneficial affect on the power output of the engine. Adisadvantage of such a system is that evaporation of the water dropletsis not readily achieved and requires onerous nozzle and spray pressurespecifications to achieve the required cooling effect.

According to one aspect of this invention, there is provided carbondioxide recirculating apparatus for an arrangement comprising combustionmeans and a path for a flow of gas through the combustion means, theapparatus comprising extraction means for extracting gaseous carbondioxide from a region of the path downstream of the combustion means,condensing means for condensing the extracted carbon dioxide, and feedmeans for feeding the condensed carbon dioxide to a region of the pathupstream of the combustion means.

The arrangement preferably comprises a heat engine or a fuel cell.

The preferred embodiment of the invention is particularly suitable foruse in an arrangement in the form of a gas turbine engine. In thisembodiment, the feed means may feed the condensed carbon dioxide to acompressor region of the gas turbine engine. The extraction means may bearrangeable downstream of a turbine arrangement of the gas turbineengine.

According to another aspect of this invention there is provided anarrangement comprising combustion means a path for the flow of gasthrough the combustion means and carbon dioxide recirculating apparatuscomprising extraction means for extracting gaseous carbon dioxide from afirst region of the path downstream of the combustion means, condensingmeans for condensing the extracted carbon dioxide, and feed means forfeeding the condensed carbon dioxide to a second region of the pathupstream of the combustion means.

The arrangement may comprise a heat engine or a fuel cell assembly. Theheat engine may be a gas turbine having a compressor region in the pathof the gas upstream of the combustion means, and a turbine region in thepath of the gas downstream of the combustion means. The compressor meansmay be a compressor unit.

The condensing means may comprise heat removal means to remove heat fromthe extracted carbon dioxide. The condensing means may includecompressor means to compress the extracted carbon dioxide. Preferably,the compressor means is arranged between the extraction means and theheat removal means. In the preferred embodiment, the heat removal meanscomprises cooling means to cool the carbon dioxide.

The feed means may comprise spray means to spray the condensed carbondioxide into the second region. Where the arrangement comprises a gasturbine engine, the spray means may spray the condensed carbon dioxideinto the compressor region of the gas turbine engine, preferably to forma fog of the carbon dioxide. The spray means may comprise atomisingmeans, which may be in the form of a nozzle. Preferably, the atomisingmeans comprises a plurality of atomising nozzles, which may be in theform of an array of nozzles.

The extraction means may comprise a recirculating amine based extractionmeans, and may include cooling and heating units to support theoperation of the amine based extraction means.

Preferably, the extraction means is arrangeable to extract carbondioxide from the exhaust gases downstream of the turbine region.

The compressor region of the engine may comprise first and secondcompressors, and the feed means may be arrangeable to feed condensedcarbon dioxide to the compressor region between the first and secondcompressors. Alternatively, or in addition, the feed means may feed thecondensed carbon dioxide to the compressor region at an outlet of thecompressor region. Alternatively, or in addition, the feed means may bearrangeable to feed the carbon dioxide at an inlet to the compressorregion.

In one embodiment, the feed means may be arrangeable to feed thecondensed carbon dioxide to the outlet of the compressor region, wherebythe carbon dioxide thereafter passes into a heat exchanger to exchangeheat with gases exiting from the turbine region of the gas turbineengine. Preferably, the heat exchanger comprises a recuperator.

The fuel cell assembly may be arranged to receive carbon dioxide fromthe carbon dioxide recirculating apparatus. The fuel cell assembly maycomprise an anode and a cathode. Preferably exhaust from the anode ispassed to the carbon dioxide recirculating apparatus.

The fuel cell assembly may comprise a compressor for compressing air andother gases to be supplied to the cathode. The fuel cell assembly maycomprise a conduit for directing recirculated carbon dioxide to theanode. Alternatively, or in addition, the fuel cell assembly maycomprise a conduit for directing carbon dioxide to the compressor formixing with air compressed by the compressor. The compressor may be acompressor of the compressor region of the gas turbine engine.

According to another aspect of this invention, there is provided amethod of recirculating carbon dioxide from a flow of gas through anarrangement comprising combustion means and a path for the flow of gasthrough the combustion means, the method comprising extracting carbondioxide from a first region downstream of the combustion means,condensing the extracted carbon dioxide and thereafter feeding thecondensed carbon dioxide to a second region upstream of the combustionmeans.

Preferably the step of condensing the extracted carbon dioxide comprisesproviding heat removal means to remove heat from the carbon dioxide andmay also include compressing the carbon dioxide prior to removing saidheat from the carbon dioxide.

The heat removal means may comprise cooling means to cool the carbondioxide to effect said condensation thereof.

The step of feeding the condensed carbon dioxide to the second region ofthe engine may comprise spraying the condensed carbon dioxide to thesecond region. The spraying of the condensed carbon dioxide may form afog of the carbon dioxide in the upstream region of the engine.

Preferably the step of feeding the carbon dioxide to the second regionof the engine comprises atomising the condensed carbon dioxide.

In the preferred embodiment, the engine is a gas turbine enginecomprising a compressor region upstream of the combustion means and aturbine region downstream of the combustion means, and the step ofextracting carbon dioxide comprises extracting carbon dioxide downstreamof the turbine region of the engine.

In the preferred embodiment, the step of feeding the carbon dioxide tothe second region of the path comprises feeding the carbon dioxide tothe compressor region of the engine. The compressor region may comprisefirst and second compressors arranged in axial flow series in the pathand the step of feeding the carbon dioxide to the compressor region maycomprise feeding the carbon dioxide between the first and secondcompressors and/or to an outlet of the compressor region and/or to aninlet of the compressor region.

The engine may comprise heat exchange means to exchange heat between gasentering the combustion means and gas exhausted from the combustionmeans, preferably downstream of the turbine region. The step of feedingthe condensed carbon dioxide to the second region may comprise feedingthe carbon dioxide to gas entering the heat exchange means upstream ofthe combustion means preferably at the outlet of the compressor region.The heat exchange means may comprise a recuperator.

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings in which:—

FIG. 1 shows a sectional view of the upper half of a gas turbine engine;

FIG. 2 is a diagrammatic representation of carbon dioxide recirculatingapparatus;

FIG. 3 is a further diagrammatic representation showing anotherembodiment of carbon dioxide recirculating apparatus;

FIG. 4 is diagrammatic representation of a further embodiment of carbondioxide recirculating apparatus incorporating fuel cell; and

FIG. 5 is a diagrammatic representation of another embodiment of carbondioxide recirculating apparatus incorporating a fuel cell.

With reference to FIG. 1, a ducted fan gas turbine engine generallyindicated at 10 has a principal axis X-X. The engine 10 comprises, inaxial flow series, an air intake 11, a propulsive fan 12, a compressorregion 113 comprising an intermediate pressure compressor 13, and a highpressure compressor 14, combustion means 115 comprising a combustor 15,and a turbine region 116 comprising a high pressure turbine 16, anintermediate pressure turbine 17, and a low pressure turbine 18. Anexhaust nozzle 19 is provided at the tail of the engine 10.

The gas turbine engine 10 works in the conventional manner so that airentering the intake 11 is accelerated by the fan to produce two airflows: a first air flow into the intermediate pressure compressor 13 anda second air flow which provides propulsive thrust. The intermediatepressure compressor 13 compresses the air flow directed into it beforedelivering that air to the high pressure compressor 14 where furthercompression takes place.

The compressed air exhausted from the high pressure compressor 14 isdirected into the combustor 15 where it is mixed with fuel and themixture combusted. The resultant hot combustion products then expandthrough, and thereby drive, the high, intermediate and low pressureturbine 16, 17 and 18 before being exhausted through the nozzle 19 toprovide additional propulsive thrust. The high, intermediate and lowpressure turbines 16, 17 and 18 respectively drive the high andintermediate pressure compressors 14 and 13 and the fan 12 by suitableinterconnecting shafts.

Referring to FIG. 2, there is shown a schematic representation of carbondioxide recirculating apparatus 20 for use in a gas turbine engine. FIG.2 shows the compressor region 113, the combustion region 115, and theturbine region 116, of the engine 10 as described above. FIG. 2 alsoshows the carbon dioxide recirculating apparatus 20 comprisingextraction means 22 arranged downstream of the turbine region 116 in themain flow of gas to be exhausted from the engine 10. The extractionmeans 22 may comprise any suitable known carbon dioxide extractionarrangement. An example of such an arrangement is a recirculating aminebased unit in which amine solvents such as diethandamine can be employedto remove the carbon dioxide to remove carbon dioxide from the gas inthe main flow downstream of the turbine region 116. Carbon dioxideextracted from the exhaust gases is passed via a line 24 to compressormeans in the form of a carbon dioxide compressor unit 26. The resultingcarbon dioxide depleted exhaust is passed to the exhaust nozzle 19.

The compressor unit 26 compresses the extracted carbon dioxide, which ispassed via a line 28 to a cooler 30 which condenses the compressedcarbon dioxide, and produces pressurised liquid carbon dioxide at ornear ambient temperature. The pressurised liquid carbon dioxide is thenpassed via a line 32 to feed means in the form of a feed nozzlearrangement 36. The pressurised liquid carbon dioxide is fed by thenozzle arrangement 36 to the compressor region 115 of the gas turbineengine 10. Specifically, the liquid carbon dioxide is fed to the mainduct designated 34 (shown schematically in FIG. 2) between theintermediate pressure compressor 13 and the high pressure compressor 14.

The feed nozzle arrangement 36 comprises an array of atomising nozzlesto spray the liquid carbon dioxide into the main duct 34 between theintermediate and high pressure compressors 13, 14.

As the liquid carbon dioxide is atomised into the duct 34, it partlyflashes to the vapour phase. Carbon dioxide which does not vaporisepartly solidifies. This results in a mixture of solid, liquid andgaseous carbon dioxide. The solid carbon dioxide then sublimes to thevapour phase, and the remaining liquid carbon dioxide then vaporises tothe vapour phase. This results in a carbon dioxide fog forming in theduct 34. The sublimation and vaporisation of the carbon dioxide absorbslatent heat of sublimation and vaporisation from the gases in the duct34 thereby cooling these gases.

FIG. 3 shows a further embodiment, which has many of the features ofFIG. 2, and these features have been designated with the same referencenumeral. In FIG. 3, the carbon dioxide recirculating apparatus isdesignated 120 and the feed means is designated 136 and comprises afirst feed nozzle arrangement 136A and a second feed nozzle arrangement136B.

The line 32 carrying the pressurised liquid carbon dioxide at ambienttemperature splits into a first line 132A leading to the first feednozzle arrangement 136A and a second line 132B leading to the secondfeed nozzle arrangement 136B. The first feed nozzle arrangement 136Aatomises the liquid carbon dioxide so that it is sprayed into the duct34 between the intermediate and high pressure compressors 13, 14. Thisspraying of the carbon dioxide has the same effects upon it as describedabove, with reference to the spraying of the carbon dioxide into theduct 34 in FIG. 2. The second feed nozzle arrangement sprays the liquidcarbon dioxide into a duct 38 downstream of the high pressure compressor14, and upstream of the combustor region 115. Again the carbon dioxidesprayed into the duct 38 undergoes the same phase changes as describedabove.

Thus, the pressurised liquid carbon dioxide sprayed into the ducts 34and 38 via the respective arrays of nozzles 136A and 136B form a carbondioxide fog in the ducts 34 and 38.

A heat exchanger in the form of a recuperator 40 is provided in theembodiment shown in FIG. 3, to exchange heat between gases exiting thecompressor region 113 and the gases exiting the turbine region 116. Thecarbon dioxide fed by the second feed nozzle arrangement 136B into themain flow of gas in the duct 38 flashes to the vapour phase, solidifies,sublimes and vaporises in the same way as described above with referenceto FIG. 2. The carbon dioxide, along with other gases in the duct 38 ispassed, as indicated by the arrow 39, to one side of a recuperator 40 toexchange heat with exhaust gases from the turbine region 116 passed tothe other side of the recuperator 40 as indicated by the arrow 42. Therecuperator 40 is provided to increase the heat in the gases enteringthe combustor 15, which also has the effect of cooling the gases exitingfrom the turbine region 116 upstream of the extraction means 22.

The exhaust gases from the turbine region 116 exit the recuperator 40and are then passed to the carbon dioxide extraction means 22 via themain duct, as indicated by the arrow 44.

FIG. 4 is a diagrammatic representation of carbon dioxide recirculatingapparatus 10 incorporating a fuel cell 50. The carbon dioxiderecirculating apparatus 20 in FIG. 4 is shown in use in a gas turbineengine 10. The features of the gas turbine engine 10 and the carbondioxide recirculating apparatus 20 are given the same reference numeralsas in FIG. 2

The fuel cell 50 is, in the embodiment shown, a fuel cell of a typeknown generally as a solid oxide fuel cell. The fuel cell 50 comprisesan anode 52 and a cathode 54.

The compressor region 113 of the gas turbine engine 10 suppliescompressed air to the cathode 54 via a recuperator 140 along a line 56.In the anode 52, the hydrogen in the fuel reacts with oxygen ionsproduced at the cathode 54 (as explained below) to produce watermolecules and electrons creating an electric current. This is anexothermic reaction and the heat generated is transferred to theincoming compressed air in the line 56 in the recuperator 140. Theoutput from the cathode 54 is passed along a line 58 (via therecuperator 140) to a combustor 160. If desired the combustor 160 can bethe combustor 15 of the engine 10.

A fuel mixture (labelled FUEL in FIG. 4) is supplied to a supplementarycompressor 62 which also received recirculated carbon dioxide from thecarbon dioxide recirculating apparatus 20 (as explained below). The fuelmixture comprises fuel, hydrogen, carbon dioxide, carbon monoxide andhydrocarbons and is compressed by the supplementary compressor 62 andfed via a line 64 to the anode 52 of the fuel cell.

The oxygen in the compressed air in the cathode 54 is electricallycharged to provide oxygen ions. The oxygen ions pass through/across thesolid oxide electrolyte membrane in the fuel cell 50 between the cathode54 and the anode 52, to react with the hydrogen in the anode 52 (asdescribed above).

The exhaust products from the anode 52 are fed via a line 66 to thecarbon dioxide extraction means 22. The carbon dioxide is extracted fromthe exhaust products and passed via the line 24 to the carbon dioxidecompressor unit 26 to be recirculated, via the cooler 30, back to thesupplementary compressor 62.

The remaining exhaust products entering the carbon dioxide extractionmeans are passed to the combustor 160 along a line 68 and fed back tothe turbine arrangement 116 of the engine 10 along a line 70. Thispowers the turbine arrangement to the drive the compressor arrangement113. The exhaust from the turbine arrangement 116 is exhausted toatmosphere via the exhaust nozzle 19, (labelled EXHAUST).

As an alternative, or in addition, as shown in broken lines, therecirculated carbon dioxide flowing along the line 28 could be fed via aline 128 to the compressor arrangement 113.

Another embodiment of a carbon dioxide recirculating apparatus 20incorporating a fuel cell 50 is shown in FIG. 5. The carbon dioxiderecirculating apparatus 20 is shown in use in a gas turbine engine 10.The features of the gas turbine engine 10 and the carbon dioxiderecirculating apparatus are given the same reference numerals as in FIG.2.

The fuel cell 50 shown in FIG. 5 is of a type known as a moltencarbonate fuel cell. In such a fuel cell 50 there is a requirement forcarbon dioxide on the air/oxygen side of the fuel cell 50, i.e. thecathode 54.

The compressor region 113 receives recirculated carbon dioxide via line128A (as explained below), in addition to air. The compressed air andcarbon dioxide is passed via the line 56 to the cathode 54 of the fuelcell 50. Some of the reaction products from the cathode are passed viathe line 58 to the combustor 160. In some embodiments, the combustor 160could be the combustor 15 of the gas turbine engine 10.

The remainder of the reaction products from the cathode 54 are passedvia a line 70 to the turbine arrangement 116, which drives thecompressor arrangement 113 as explained above. The exhaust 72 from theturbine arrangement 113 drives a free power turbine 74, which, in turn,drives a further compressor 76 via a shaft 78.

The exhaust from the free power turbine 74 is passed via a line 90 to aheat exchanger or recuperator 92 where heat is exchanged with compressedgases exiting from the compressor arrangement 113 prior to entering thecathode 54.

After exiting the recuperator 92, the gases from the free power turbine74 are exhausted to atmosphere via the exhaust nozzle 19.

The combustion products from the combustor 160 are passed via a line 80to the further compressor 76. The further compressor 76 also receivesrecirculated carbon dioxide (as explained above) via a line 128B. Thecompressed combustion products and carbon dioxide are passed to thecathode 54 via a line 82.

The anode 52 receives fuel via the line 64. The reaction products passfrom the anode 52 by a line 66. Some of the reaction products may berecirculated via a line 84 and a supplementary compressor 86 to bepassed back into the anode 52.

The reaction products from the anode 52, which are not recirculated, aresplit into two. Some of the reaction products from the anode 52 arepassed via a line 66A to the combustor 160 and are mixed with theincoming reaction products from the cathode 54 and combusted. Theremainder of the reaction products from the anode 52 are passed via aline 66B to the carbon dioxide extraction means 22.

The carbon dioxide is extracted and passed via the line 24 to the carbondioxide compressor unit 26 and then cooled by the cooler 30. The cooledcarbon dioxide exits the cooler 30 via the line 128. Some of the cooledcarbon dioxide is passed via the line 128A to the compressor arrangement113. The remainder of the cooled carbon dioxide is passed via the line128B to the compressor 76, as described above.

The remaining cathode reaction products in the carbon dioxide extractionmeans 22 are passed via the line 68 to the combustor 160 to becombusted.

The above described embodiments have the advantage that the fog sprayedinto the compressor region is formed from liquid carbon dioxide thatflashes under more favourable and more easily achieved conditions thanwater. As a result, a fog with a small droplet size is generated morereadily than with water. This results in there being less demandingnozzle and spray pressure specifications than are necessary with water.

Further, the above described embodiments have the advantage that the useof carbon dioxide means that complete evaporation of the fog can be moreeasily achieved than with water, due to the high saturated vapourpressure of the carbon dioxide at ambient temperatures. This permits theuse of carbon dioxide based fog cooling to be used in conditions thatwould be too confined in length to achieve adequate evaporation with awater based fog. A further benefit is that carbon dioxide recirculationis achieved with less compression in the recirculating system than witha water based recirculation. It is possible thus to enhance efficiencyand power of the engine more substantially than with the use of water.

The use of carbon dioxide has the further advantage that cooling couldbe carried out at the inlet of the compressors of a gas turbine engine,and could also be used in other engines, for example reciprocatingengines. Indeed, the invention could be applied to a wide range ofcycles using heat engines and/or fuel cells as the primary source ofcarbon dioxide.

Whilst endeavouring in the foregoing specification to draw attention tothose features of the invention believed to be of particular importanceit should be understood that the Applicant claims protection in respectof any patentable feature or combination of features hereinbeforereferred to and/or shown in the drawings whether or not particularemphasis has been placed thereon.

1. Carbon dioxide recirculating apparatus for an arrangement having acompressor region upstream of a combustion means, a turbine regiondownstream of said combustion means and an exhaust nozzle downstream ofsaid turbine region, the apparatus being arranged to supply air to thecompressor region and to supply fuel to the combustion means whereinsaid fuel is burned in said combustion means producing exhaust gases,the apparatus having means for extracting carbon dioxide from saidexhaust gases from a first region downstream of the turbine region,condensing means for condensing the extracted carbon dioxide, and feedmeans for feeding the condensed carbon dioxide to a second regionupstream of the combustion means without adding any constituent thereto,said second region comprising the compressor region.
 2. Carbon dioxiderecirculating apparatus according to claim 1 wherein the condensingmeans comprises heat removal means, to remove heat from the extractedcarbon dioxide, and compressor means to compress the extracted carbondioxide, the compressor means being arranged between the extractionmeans and the heat removal means.
 3. Carbon dioxide recirculatingapparatus according to claim 2, wherein the heat removal means comprisescooling means to cool the compressed carbon dioxide.
 4. Carbon dioxiderecirculating apparatus according to claim 1, wherein the feed meanscomprises spray means to spray the condensed carbon dioxide into thesecond region of the path to form a fog of the carbon dioxide.
 5. Carbondioxide recirculating apparatus according to claim 4, wherein the spraymeans comprises atomising means.
 6. Carbon dioxide recirculatingapparatus according to claim 5, wherein the atomising means comprises anozzle.
 7. A combustion arrangement comprising combustion means, a pathfor a flow of gas through the combustion means and carbon dioxiderecirculating apparatus as claimed in claim
 1. 8. An arrangementaccording to claim 7, wherein the arrangement is in the form of a heatengine or a fuel cell.
 9. An arrangement according to claim 8, whereinthe heat engine comprises a gas turbine engine having a compressorregion in the path upstream of the combustion means and a turbine regionin the path downstream of the combustion means.
 10. An arrangementaccording to claim 9, wherein the extraction means is arrangeable toextract carbon dioxide from the exhaust gases downstream of the turbineregion.
 11. An arrangement according to claim 9, wherein the feed meansis arrangeable to feed condensed carbon dioxide to the compressorregion.
 12. An arrangement according to claim 9, wherein the compressorregion comprises first and second compressors arranged in the path inaxial flow series, and the feed means is arranged to feed the condensedcarbon dioxide to one or more of: the inlet to the compressor region;between the first and second compressors; and the outlet of thecompressor region.
 13. An arrangement according to claim 9, wherein theengine includes a heat exchanger to exchange heat between gas enteringthe combustion means and heat exhausted from the combustion means, andthe feed means is arranged to feed at least some of the condensed carbondioxide to the outlet of the compressor region upstream of the heatexchanger.
 14. An arrangement according to claim 1, wherein thecompressor region comprises first and second compressors and the feedmeans is arranged also to feed some of the condensed carbon dioxidebetween the first and second compressors and/or to the inlet of thecompressor region.
 15. A combustion arrangement according to claim 7including a fuel cell assembly to receive carbon dioxide from the carbondioxide recirculating apparatus.
 16. A combustion arrangement accordingto claim 15, wherein the fuel cell assembly comprises a cathode toreceive carbon dioxide from the carbon dioxide recirculating apparatusand an anode, the exhaust from the anode being passed to the carbondioxide recirculating apparatus.
 17. A combustion arrangement accordingto claim 16 comprising a compressor for receiving recirculated carbondioxide and for compressing gases prior to said gases being passed tothe cathode.
 18. A combustion arrangement according to claim 16, whereinrecirculated carbon dioxide is passed to the anode.
 19. A combustorarrangement according to claim 17 including a turbine driven by gasesexhausted from said cathode, the turbine being arranged to drive thecompressor.
 20. A combustor arrangement according to claim 19, whereinthe turbine comprises a free power turbine driven by gases exhaustedfrom a principal turbine.
 21. A combustor arrangement according to claim20, wherein the principal turbine is driven by gases exhausted from thecathode, and the arrangement further includes a principal compressordriven by the principal turbine, the principal compressor receivingrecirculated carbon dioxide and compressing gases prior to said gasesbeing passed to the cathode. 22-32. (canceled)
 33. A method ofrecirculating carbon dioxide from a flow of gas through a gas turbineengine comprising a compressor region upstream of a combustion means, aturbine region downstream of said combustion means and an exhaust nozzledownstream of said turbine region, the method comprising supplying airto the compressor region, supplying fuel to the combustion means,burning said fuel in the air in said combustion means to produce exhaustgases, extracting carbon dioxide from the exhaust gases from a firstregion downstream of the turbine region, supplying carbon dioxidedepleted exhaust gases to the exhaust nozzle, condensing the extractedcarbon dioxide and thereafter feeding the condensed carbon dioxide to asecond region upstream of the combustion means without adding anyconstituent thereto, the second region comprising the compressor regionwherein the method utilizes a heat engine or a fuel cell.